In general, a vehicle occupant restraint air bag inflator system includes a pressure vessel, within which pressurized and expandable gas may be stored, and a means for rapidly expanding the gas so that the gas is expelled from the vessel into an expandable air bag in the event of an accident. One type of air bag inflator system, referred to as a hybrid system, includes a pressure vessel within which an inert gas, under pressure, is stored. A solid propellant material is stored within a suitable container at one end of the vessel. The opposite end of the vessel, which is normally sealed, is connected to the deflated air bag. In the event of an accident, that is, a sudden stop of the vehicle, a sensor acts to ignite the propellant material which, in turn, discharges into the stored pressured inert gas in the vessel to suddenly and rapidly heat the gas and cause it to expand. The expanding gas discharges through the opposite, outlet end of the vessel, into the deflated air bag. The sudden burst of expanding gas almost instantly inflates the air bag to protect the vehicle occupant and to hold the occupant against moving forwardly and striking the vehicle interior surfaces.
An example of a commercially known hybrid-type air bag inflator includes an elongated, tubular pressure vessel for containing an inert gas, such as argon gas. A solid propellant that produces gas, such as sodium azide, is arranged at one end of the inert gas-containing vessel and is separated from the interior of the vessel by an openable closure. The opposite end of the vessel is closed by a pressure sensing closure which will open upon a threshold pressure reached within the vessel. A suitable ignitor actuates the solid propellant upon receipt of a signal from a device which senses a rapid, crash-type stop of the vehicle within which the inflator is installed. Upon such ignition, the solid propellant produces the above mentioned gas, e.g., sodium azide, which bursts through the closure to the interior of the pressure vessel and heats the inert, argon gas within the container. That causes the inert gas to rapidly expand and to exceed the pressure needed to open the closure at the outlet end of the vessel. The expanding gas is directed into the deflated air bag and causes the air bag to almost instantly inflate in front of the vehicle occupant.
The particular structure described above is normally intended for use within an automotive vehicle at the passenger side of the vehicle. That is, the tubular vessel, which may be formed of an elongated tube, of the order of 3-7 inches in length, for example, is intended to be positioned within the dash board of the vehicle in front of the passenger. The same inflator system may be used elsewhere, but it is particularly useful for the front passenger side of a vehicle as well as for the occupants of the rear seats of the vehicle. The driver side of the vehicle normally requires a different type of vessel shaped to be fitted within the steering column of the vehicle.
With the foregoing tube shaped pressure vessel, it is desirable to use a seamless, thin walled, strong tube. Although bursting of the tube is unlikely, it is necessary that the vessel have sufficient strength to avoid unintended bursting caused by a gas pressure build-up within the vessel which exceeds the burst strength of the tube. In the event of unintended bursting, the vessel could fragment, creating safety and damage problems within the vehicle.
At this time, the method for producing the required vessel tube generally involves stamping and drawing a thin, flat, metal plate into a cup-shape and then elongating the cup-shape through successive stamping or drawing steps. In this process, the cup-shape is successively drawn deeper and deeper to form the final, elongated, seamless tube shape. This requires repeated handling of the part during the successive steps in the process, as well as annealing between steps. Moreover, to avoid the possibility of the finished vessel bursting and fragmenting under excessive pressure, a relatively thick steel sheet must be used. This increases the cost of the tube and increases the weight, as well as increases the level of difficulty in forming the tube.
Because air bag inflators are made in high-volume production, a manufacturing procedure which reduces the costs of manufacturing each vessel even a very small amount or which permits a reduction in the thickness of the metal used even a slight amount or which permits the use of a less expensive metal material, is highly desirable. That is, the high volume would provide a substantial total cost savings. Thus, this invention is concerned with providing a method and an improved pressure vessel tube which will increase the strength of the tube, prevent fragmentation of the tube in the unlikely event of bursting, and substantially reduce the costs of production, including permitting the use of less expensive materials.
This invention contemplates adapting a previously known extrusion procedure for making hollow tubes or shafts to make pressure vessel tubes for air bag inflators. In this procedure, ring-like metal blanks are pushed through a constricted die throat or orifice by a suitable punch. A mandrel-like extension on the punch, arranged within the ring-like blank, helps form the extruded, hollow, thin-walled, elongated tube. Such tubes are produced rapidly, one-by-one, and may be provided with either a uniform wall thickness throughout their lengths or with thickened wall sections at selected portions thereof, such as at their opposite ends. Examples of this method for forming tubes are illustrated, for example, in U.S. Pat. Nos. 4,002,286 issued Jan. 11, 1977; 4,277,969 issued Jul. 14, 1981; 4,282,831 issued Oct. 6, 1981; 4,435,972 issued Mar. 13, 1984; and, 4,991,421 issued Feb. 12, 1991.
This present invention, by utilizing a tube extrusion process, enables the formation of relatively short, small diameter, thin walled, tubular pressure vessels which are incorporated in a gas inflator system for inflating passenger restraint air bags.